트롤어구의 소형어류 탈출장치 개발

Abstract

The aims of this study were to develop a Trawl Escapement Device(TED) to improve the survival rate of undersized fishes that escape from trawls. The first sea experiments were conducted to investigate the survival rate of fishes discarded after hauling and to develop fishing gears and methods for fisheries management which can improve the survival rate of undersized fishes escaping from the grid and codend.The trials were carried out in offshore waters of the Korean southern sea and off Jeju Island. Fish were placed in a fish cage on board to measure the sustainable survival time. Japanese flying squid (Todarodes pacificus), mitra squid (Loligo chinesis), large-head hairtail (Trichiurus lepturus), john dory (Zeus japonicus), Spanish mackerel (Scomberomorus niphonius), redlip croaker (Pseudosciaena polyactis) and blackthroat seaperch (Doederliinia berycoides) were found to be dead immediately after hauling. Survival rates of tiger shark (Scyliorhinus torazame),conger eel (Conger myriaster), red skate (Dasyatis akajei), black scraper(Navodon modestus) and Japanese fan lobster (Ibacus ciliatus) were estimated to be higher after discarding because they survived for long hours in the fish cage. Blotched eelpout (Zoarces gilli) escaped from the escapement device (Grid) were dead within 6 hours in the water tank but 97% of tiger sharks and 72% of conger eels survived over 72 hours.Red skate that escaped from trawl codend were dead within 60 hours in the water tank but 25% of octopus (Paroctopus dofleini) and all black scrapers survived over 72 hours. All rosefish (Helicolenus higendorfi)that escaped from the codend were dead within 72 hours but 8.3% of the rosefish that escaped from the escapement device survived over 72 hours. A second set of sea experiments was carried out to investigate the survival rate of the fishes that escaped from the TED and codend in off Geomundo and in the Korean southern sea. A cover-net fish cage was designed and manufactured to collect fishes that escaped from the TED and codend. The fish cage was installed at a shallow site where a scuba diver could observe the surviving fishes in a fish cage separated from the trawl net. The survival rate of horse mackerel escaped from the TED was 85.5% after 96 hours while the survival rate for fish from the codend showed a rapid decrease according to the time elapsed as follows:85.0% survival after 24 hours, 76.4% after 48 hours, 62.7% after 72 hours and 56.1% after 96 hours. Over 90% of red seabream (Pagrus major)and rock bream (Oplegnathus fasciatus) that escaped from the TED survived 96 hours and the survival rate of those fishes was excellent compared with the other fish species. All Korean pomfret that escaped from both TED and codend died within 48 hours. Most of redlip croaker also died within 12 hours because of their limited physiological ability to withstand water pressure change. The survival rate of fishes such as horse mackerel with hard skin was high while survival was low for soft-skinned species like the Japanese flying squid and large-head hairtail. A third set of sea experiments was conducted to develop a Trawl Escapement Device (TED) which can improve the escapement rate of undersized fishes that pass through a trawl net. The sea experiments involved varying the bar space of the device and the bar materials to identify the best bar space and materials. Trials were carried out in the Korean southern sea and off Jeju island. Stainless steel pipes and four kinds of plastics were used to examine the efficiency of different escapement device materials. Although escapement rates from the MC bar for the main species were a little higher compared with those stainless steel pipes, stainless steel pipe was chosen as the material for the TED. The escapement rate for sort-x type and grid panel type were also compared. Escapement rates of red-lip croaker, black-throat seaperch and finespotted flounder (Pleuronichthys cornutus) were 61.7%,2.0% and 2.7% respectively for the sort-x type and 69.9%, 1.1% and 2.0% for the grid panel type, a relatively small difference. Escapement rates in relation to the bar space were estimated using the grid panel type. The survival rates for redlip croaker for the bar space of 20mm,25mm, 35mm were 60.3%, 61.0% and 77.8% respectively, and 9.8%, 16.4% and 45.6% for horse mackerel (Trachurus japonicus). The fourth set of experiments for the study was carried out to develop a Trawl Escapement Net (TEN) using net material in the Korean southern sea in May and November, 2005. Three kinds of Trawl Escapement Net (TEN) were examined to improve the escapement rate for undersized fishes escaped from the TEN, and catch numbers of escaped fishes were investigated. The main catch species in the experimental fishing trials using a V shape TEN (A type) in May were Korean pomfret (Pampus echinogaster), slimy (Leiognathus nuchalis),gizzard shad (Konosirus punctatus), white croaker (Pennahia argentata) and smelt (Sillago sihama). In the experimental fishing trials using a V shape TEN with a protector for undersized fishes (B type) and a panel shape TEN with a protector (C type) in November, the main catch species were horse mackerel, sea pike (Sphyraena japonica), large-head hairtail, kammal thryssa (Thryssa kammalensis), white croaker, butterfish(Psenopsis anomala) and red gurnard (Lepidotrigla microptera). Most of large-sized fish species like Korean pomfret and Spanish mackerel could not escape through the TEN in the B type and C type. Catches of horse mackerel were highest in the experimental fishing of November and the escapement rate of horse mackerel was also highest, at 38.3%, and escapement rates for most fish species were higher in the C type compared with B type.